Radiation detector with remote temperature reference

ABSTRACT

A radiation detector employs a thermopile having a potentiometer for scaling the thermopile output to best suit a particular output meter and sensing application. The output of the thermopile is scaled to approximate the output of a thermocouple in contact with a target surface and is indicative of the temperature of the target. Two detectors may be connected differentially to provide a differential output indicative of the temperature difference between two targets. Additionally, a temperature dependent variable resistor may be coupled to the thermopile, providing a variable resistance that combines with the thermopile output response to produce a linearized thermopile output response. Then, the total output signal of the detector for a particular target temperature is independent of fluctuations in local temperature.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/254,126filed Jun. 6, 1994, U.S. Pat. No. 5,528,041, which is a CIP of08/041,933, filed Apr. 2, 1993, U.S. Pat. No. 5,319,202, which is aContinuation of 07/716,038, filed Jun. 14, 1991, U.S. Pat. No.5,229,612, which is a CIP of 07/561,169, filed Aug. 1, 1990, nowabandoned.

BACKGROUND

Quality control of a product or process has become a large part of theeconomics of industry. A major concern of quality control is accuracy inmeasuring and the ability to detect the slightest fault in a variety ofproducts and processes. Various devices are used to measure differencesin weight, temperature and other dimensions. Such devices are usuallynonportable, time consuming, inaccurate, invariable for use in detectingmore than one object, and often incapable of giving a quantitativeanalysis.

Radiation detectors can be used to detect abnormalities by measuringtemperature change and heat loss or gain. Radiation detectors have beenused as a non-contact alternative to many temperature sensors. Infraredscanning devices have also been used to detect temperature differencesbetween a subject and a reference as well as to measure heat loss frommachinery, plumbing, electrical lines and the like. Typically suchradiation detectors and infrared scanning devices employ radiationsensors which respond to changes in radiation in the order of 1/10second. Such sensors are not only fast, but accurate and economic asoperations of interest do not need to be shut down during detection.

Radiation detectors are based on the principle that the thermalradiation emitted from a subject is proportional to the temperature ofthe subject raised to the fourth power. The radiation emitted is also afunction of the emissivity of the subject and of background radiation,but can be calibrated out for applications in which the target hasconsistent properties.

One type of radiation sensor is a thermopile. Thermopiles in generalhave been used to provide an indication of target temperature. Athermopile operates on the principle that sensed radiation causes avoltage to be produced at the thermopile output which is indicative ofthe difference between the hot and cold junctions of the thermopile.

One typical problem with radiation sensors such as thermopiles is theirtendency to become overheated by energy trapped within the device. Suchoverheating and retaining of energy by the radiation sensor causesinaccuracies in the temperature readings. Many sensing applicationsrequire close range detection. A user in such a situation often runs therisk of heating or cooling the device with changing environmentalconditions, which may change the cold junction temperature of the deviceor perhaps even distort the sensor output by causing uncontrolledthermal gradients. In addition to heat management problems, radiationsensor devices face dirty as well as harsh environments. Elaboratecooling, purging and cleaning systems have been used, but are expensive,clumsy and require maintaining close calibration.

SUMMARY OF THE INVENTION

Provided with the present invention is a radiation detector having athermopile sensing radiation emitted from a target, and providing anoutput signal indicative of the temperature of the target. To allowcalibration of the thermopile output signal, a calibrator such as apotentiometer or other variable resistance is provided at the thermopileoutput. By enabling a user to adjust the potentiometer, the thermopileoutput signal may be user scaled to calibrate the output signal tointersect a thermocouple output response at a desired targettemperature.

Although the thermopile and potentiometer together form a detector whichcan be adjusted to suit a particular application, a preferred embodimentalso has a thermocouple which provides an output signal that combineswith the output signal of the thermopile to produce a total outputsignal. To provide compensation for output changes due to changes inlocal temperature, the change in the thermopile output signal with achange in the local temperature is inversely related to the change inthe thermocouple output signal with a change in the local temperature.

By connecting the thermocouple electrically in series with thethermopile, the output voltages of the thermopile and the thermocouplecombine to provide a total output voltage. The hot junction of thethermocouple is held at the cold junction temperature of the thermopile.Thus, with the thermopile thermal response to the common junction localtemperature being close to the inverse of the thermocouple thermalresponse to the local temperature, changes in the total output signalare substantially independent of fluctuation of the temperature at whichthe thermocouple hot junction and the thermopile cold junction are held.

In one embodiment, a lens is provided for filtering out shorterwavelengths from the radiation sensed by the thermopile. This helpsimprove the linearity of the thermopile thermal response in a targettemperature range of interest. With the total output response of thesensor approximating a linear function in a temperature range ofinterest, a linear output means such as a meter responsive to linearinputs may be controlled directly from the total output signal.

In another embodiment, the filter passes shorter wavelengths,substantially filtering out longer wavelengths such as those greaterthan 6 microns. Although such a sensor loses linearity, it issignificantly less sensitive to changes in emissivity with change intemperature over a narrow target temperature range. Accordingly, such adevice is particularly suited to low emissivity targets.

The cold junction temperature to which the hot junction temperature ofthe thermopile is referenced is at the local hot junction temperature ofthe thermocouple which is referenced to the thermocouple cold junction.The thermocouple cold junction reference temperature may be locatedremote from its hot junction and the thermopile sensor. This preventschanges in output of the sensor due to incidental heating of the localreference temperature due to its proximity to the target.

one embodiment of the present invention provides for a differentialradiation detector. In that embodiment, a first thermopile sensesradiation from a first target and provides an output signal indicativeof the temperature of the first target. A first thermocouple provides anoutput signal which combines with the output signal of the firstthermopile to produce a first total output signal. A change in theoutput signal of the first thermopile with changes in a first localtemperature is inversely related to a change in the output signal of thefirst thermocouple with changes in the first local temperature.

In addition to the first thermopile/thermocouple combination, a secondthermopile senses radiation from a second target and provides an outputsignal indicative of the temperature of the second target. A secondthermocouple provides an output signal which combines with the outputsignal of the second thermopile to produce a second total output signal.A change in the output signal of the second thermopile with changes in asecond local temperature is inversely related to the change in theoutput signal of the second thermocouple to changes in the second localtemperature. The cold junction of the first thermocouple and secondthermocouple are held to a common temperature and thethermocouple/thermopile pairs are coupled to provide a differentialoutput. Calibrators and lenses may also be provided in the same manneras with the single thermopile sensor embodiment. It is preferable thatthe thermopiles are matched and the thermocouples are matched to providean accurate differential response.

In accordance with another embodiment of this invention, a radiationdetector has a temperature dependent variable resistor coupled to thethermopile and providing a variable resistance that combines with thethermopile output voltage to produce a linearized thermopile outputvoltage. As such, the thermopile output, linearized by the thermistor,combines with the linear thermocouple output to provide a detectoroutput that is more stable with changes in the thermopile cold junctiontemperature.

In the aforementioned embodiments, the thermopile and the thermocoupletogether form a detector suitable for applications for an expected meantarget temperature and within a common junction local temperature range.However, since the linear thermal response of the thermocouple isemployed to compensate for the non-linear thermal response of thethermopile, the local temperature range of the common junction must beknown and relatively narrow. Accordingly, the primary advantage of thisembodiment is that detector output less dependent on the thermopile coldjunction temperature over a broad range.

Accordingly, a thermocouple is connected electrically in series with thethermopile/thermistor circuit such that changes in the thermocoupleoutput voltage due to changes in thermopile cold junction temperatureare inversely related to changes in the linearized thermopile outputvoltage due to said changes in thermopile cold junction temperature.Thus, this embodiment utilizes the thermocouple, which provides a linearthermocouple output, to compensate for the linearized thermopile outputwith changes in the cold junction temperature of the thermopile, therebymaintaining a stable detector output voltage for a given targettemperature. Since the thermocouple is connected in series with thethermopile/thermistor circuit, the remote thermocouple cold junctionbecomes the thermopile reference. As such, there is no need to measurethe thermopile cold junction temperature or to force the cold junctiontemperature into a particular range.

The temperature dependent variable resistor preferably comprises atleast one negative temperature coefficient (NTC) thermistor electricallyconnected in series with the thermopile and thermally coupled to thecold junction of the thermopile. To achieve linearization of thethermopile output voltage over a thermopile cold junction temperaturerange, an NTC thermistor is selected wherein the change in theresistance of the thermistor due to a change in thermopile cold junctiontemperature modifies the thermopile output response in a manner that isinversely related to the change in the thermopile output voltage withsaid change in a thermopile cold junction temperature. In an alternativeconfiguration, at least one positive temperature coefficient thermistormay be electrically connected in parallel with the thermopile andthermally coupled to the thermopile cold junction. In either case, theresulting thermopile output voltage is a more linear function withchanges in the thermopile cold junction temperature.

This embodiment of the present invention is particularly useful inapplications in which the target temperature is known and relativelystable. Depending on the target temperature range of interest, differenttypes of thermistors or even multiple thermistors may be used incombination with standard resistors to provide for linearization of thethermopile output voltage over a wide range of thermopile cold junctiontemperature variations.

As in previous embodiments, a calibrator such as a potentiometer may beemployed to fine-tune the linearized thermopile output response tointersect a thermocouple output response at a desired target temperatureto produce a stable detector output for a thermopile cold junctiontemperature range of interest. Also, since thermopiles have parametersthat vary significantly from device to device, the potentiometer may beadjusted to compensate for these variations such that a number ofdevices may be tuned to provide the same detector output for the desiredtarget temperature.

In accordance with another aspect of the present invention, thethermocouple may comprise a nonintersecting pair of leads formed ofdifferent thermocouple materials and coupled to a thermopile circuitsuch that the thermopile circuit actually serves as the hot junction ofthe thermocouple. For the thermocouple to be electrically connected inseries with the thermopile, a first thermocouple lead is electricallyconnected to one of a pair of thermopile leads which are connected to athermopile circuit and therefore held at the cold junction temperatureof the thermopile. Although the second thermocouple lead is notelectrically connected directly to a thermopile lead, it is electricallyconnected to the thermopile circuit. Further, the second thermocouplelead is mounted in close proximity to the thermopile and thermallycoupled to the cold junction of the thermopile with epoxy. With boththermocouple leads held at the same temperature, the temperature of thethermopile cold junction, the leads do not have to intersect to providea thermocouple hot junction.

A meter may be coupled to the detector output. The meter may be of atype typically used to measure a thermocouple output. Since both themeter and the thermopile circuit are high impedance devices, thethermopile acts as an antenna receiving stray high frequency noise whichdistorts the meter measurement. In accordance with the presentinvention, a filter is coupled to the thermopile to attenuate highfrequency noise, specifically noise at and above 60 Hz. Preferably, thefilter comprises a capacitor having a value of 1-5 μf and which isconnected in parallel with the detector output. At high frequencies, thecapacitor causes the thermopile circuit output impedance to be lowthereby eliminating the presence of high frequency noise at the meter.

In another embodiment of the present invention, a radiation detectorcomprises a thermopile and a thermistor and provides a linearizedthermopile output voltage. Since the linearized thermopile output is alinear function with changes in the thermopile cold junctiontemperature, a linear output means with linear cold junctioncompensation may be coupled to the detector to provide temperatureindications.

In yet another embodiment of the present invention, a temperaturemonitoring system monitors the temperature of a product positioned in aprocess chamber. The monitoring system comprises a thermopile whichsenses radiation emitted by the product and provides a thermopile outputsignal indicative of the product temperature. Preferably, a thermocoupleand a temperature dependent variable resistor are electrically andthermally coupled to the thermopile to provide an output signalindicative of the product temperature. As long as the producttemperature remains within acceptable limits, the output signal is alinear function over a product temperature range of interest and isindependent of fluctuations in local temperature.

The monitoring system also comprises a thermal heat sink having a firstend extending into the process chamber and having a second end disposedin an ambient temperature environment. The heat sink may comprise acopper pipe or a heat pipe. The radiation detector is thermally coupledto the heat sink adjacent to the first end to view the product. Sincethe components within the radiation detector have a maximum localoperating temperature which may be less than the temperature of theprocess chamber, the temperature of the heat sink adjacent to thedetector does not exceed the maximum operating temperature of thecomponents within the detector.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention

FIG. 1A is a diagram of the electrical circuit of a thermopile radiationdetector embodying the present invention.

FIG. 1B shows a circuit similar to the circuit of FIG. 1A, but having athermocouple in series with a thermopile.

FIG. 2 is a graph of thermal response to sensed radiation of thethermopile of FIG. 1.

FIG. 3 is a graph of the relative percentage of total emitted radiationreaching the thermopile versus target temperature.

FIG. 4 is a diagram of an electrical circuit of a differential radiationdetector of the present invention.

FIG. 5A is a diagram of an electrical circuit of a thermopile radiationdetector embodying the present invention.

FIG. 5B is a diagram of an alternative electrical circuit of athermopile radiation detector of FIG. 5A.

FIG. 6A is a graph of thermal response to sensed radiation of thethermopile of FIG. 1.

FIG. 6B is a graph of the thermal response to cold junction temperaturefor a fixed target temperature of the thermopile of FIG. 1.

FIG. 6C is a graph of the thermal response for a fixed targettemperature of a temperature dependent variable resistor of theradiation detector of FIG. 1.

FIG. 7 is a plan view of the radiation detector of FIG. 5A.

FIG. 8 is a diagram of an electrical circuit of a alternative embodimentthermopile radiation detector of the present invention.

FIG. 9 is an electrical circuit diagram of yet another embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Shown in FIG. 1A is a radiation detector which uses a thermopile 19 tosense radiation emitted from a target. A potentiometer 17 is connectedacross the output leads of thermopile 19 to provide a means by which tocalibrate the thermopile output. As further described below, thiscalibration enables a final output voltage, and hence, a targettemperature displayed by a readout device 10, to be closely related tosensed radiation in a temperature range of interest. In a preferredembodiment, potentiometer 17 is a 100 KΩ trimpot.

A thermopile is defined to produce across its two ends a voltageproportional to the temperature difference of a series of hot and coldjunctions between the two ends of the thermopile. Thus, the outputvoltage (E_(p)) of thermopile 19 can be represented by the relationship

    E.sub.p =α.sub.p N(T.sub.1 -T.sub.2)K                (1)

where T₁ is the temperature of the hot junctions 8 of thermopile 19, T₂is the temperature of the cold junctions 9 of thermopile 19, α_(p) is aSeebeck coefficient for the thermopile materials, N is the number of hotand cold junctions 8 and 9, and K is a scaling coefficient due to thepotentiometer 17.

In general, the relationship between target temperature T_(T), andthermopile output voltage E_(p) may be shown by the following equation:

    E.sub.p =α.sub.p N(T.sub.1 -T.sub.2)K=α.sub.p 'e.sub.t σ(T.sub.T.sup.4 -T.sub.1.sup.4)K                    (3)

Where α_(p) ' is a Seebeck coefficient for the thermopile in units ofvolts/BTU-hr-ft², e_(T) is the emissivity of the target surface, and ais the Stefan-Boltzmann constant. The coefficient α_(p) ' can be furtherdefined by the following relation:

    α.sub.p '=(1+c(T.sub.2 -T.sub.R))α.sub.pr '    (4)

where c is a Seebeck temperature coefficient for the thermopile, α_(pr)' is the value of α_(p) ' selected for a specific thermopile, coldjunction reference temperature, and T_(R) is the reference temperaturefor which α_(pr) ' is selected. For practical applications T_(R) ischosen as the expected value of T₂.

Substituting equation (4) into equation (3), the thermopile outputvoltage may be represented as

    E.sub.p =(1+c(T.sub.2 -T.sub.R))α.sub.pr 'e.sub.T σ(T.sub.T.sup.4 -T.sub.1.sup.4)K                    (5)

As shown by equation (5), the thermopile output voltage E_(p) willchange nonlinearly with changes in T_(T) and T₁ due to the presence ofthe fourth power term (T_(T) ⁴ -T₁ ⁴). This fourth power term isdemonstrated by the curve 14 of FIG. 2 which shows how the radiationsensed by the thermopile 19 (in BTUs) increases with increasing targettemperature. FIG. 2 assumes a thermopile cold junction temperature of T₂=70° F., and the BTU scale of the curve therefore represents BTUs above70° F.

Since the output device 10 is typically a standard meter which respondsto a linear output function, such as is produced by a thermocouple, itis desirable to have the thermopile output voltage E_(p) appear as muchlike a linear function of target temperature as possible. Shown in FIG.2 with curve 14 are linear calibration lines showing a linear change insensed radiation with changing target temperature, as would be desiredwhen using a linear output device. Each calibration line is shownintersecting curve 14 at a different point. Within a given range oftarget temperatures around a particular intersection point, the curve14, representative of the thermopile output, is a good approximation ofthe desired linear response. Within the given temperature range, theapproximation is sufficient to allow the construction of an accuratetemperature detector having a thermopile sensor and a linear outputmeter.

For design purposes, it is difficult to anticipate the temperature ofthe thermopile hot junction T₁. However, for most practicalapplications,

(T_(T) -T₁)>>(T₁ -T₂)

Therefore, for the purposes of establishing the value of thermopileoutput voltage E_(p), the approximation T₁ ≈T₂ can be used. Making thesubstitution of T₂ for T₁ in equation (5), the thermopile output voltagebecomes:

    E.sub.p =(1+c(T.sub.2 -T.sub.R))α.sub.pr 'e.sub.t σ(T.sub.T.sup.4 -T.sub.2.sup.4)K                    (6)

From equation (6), knowing the expected mean target temperature, and theexpected cold junction temperature allows equation (6) to be manipulatedto build a detector having an output response which intersects with alinear approximation function at the desired target temperature. Formany applications, such a detector is sufficiently accurate within arange of target temperatures around the expected mean targettemperature, and thereby functions as a practical detection device.However, for a given application, a means for calibrating the outputvoltage E_(p) to the desired approximation is required.

The calibration of the thermopile response can be partially accomplishedby selecting a thermopile having a value of σ_(pr) ' such that thethermopile output best achieves the desired intersection point betweenthe thermopile curve and the chosen linear approximation. However, dueto the limited types of thermopile materials available, the value ofα_(pr) ' often can not be selected as desired, in which case anintersection point at a desired target temperature can not be met. Thepresent invention therefore provides potentiometer 17 which scales theof the thermopile, allowing the output voltage E_(p) detector to be"fine-tuned" to the expected mean target temperature. This ensures thatthe approximation is as accurate as possible. Having the manuallyadjustable potentiometer 17 also allows a user of the detector tocorrect for any inaccuracies of the detector due to manufacturingtolerances or other influences. The presence of potentiometer scalingfactor K in equation (6) shows that the thermopile output voltage E_(p)can be controlled by controlling the setting of the potentiometer.

FIG. 1B illustrates a further embodiment of a radiation detector 12 thatuses the thermopile 19 and potentiometer 17 of the circuit of FIG. 1A,as well as a thermocouple 15. Thermal radiation emitted from a targetenters detector 12 through window 11 of thermopile assembly 30 and isreceived in assembly 30 by thermopile 19. Thermopile 19 is connected inseries to thermocouple 15 in a manner such that a final output voltageindicative of sensed radiation is provided across the ends of the leadwires 23 and 25 of thermocouple 15. These ends are connected torespective terminals 21 and 27 of readout device 10 which uses the finaloutput voltage to provide an indication of target temperature as afunction of sensed radiation.

The thermocouple 15 of the present embodiment increases the accuracy ofthe detector by providing compensation to the thermopile sensor forchanges in thermopile cold junction temperature T₂. Rather than forcingT₂ to equal a particular temperature, or measuring T₂ to use incalculating the final meter output, the present invention usesthermocouple 15 to automatically compensate for fluctuation in T₂, whileproviding a reference temperature to the detector which is remote fromthe thermopile and the target.

A thermocouple is defined to produce a voltage between two junctionsformed by two dissimilar metal wires connected to each other at theirends where one junction is at a different temperature than the secondjunction. Thermocouple 15 of FIG. 1B is made of wire 25 and wire 23.Wire 23 as shown by the broken line is of a different metal than wire25. Effectively, the two junctions of thermocouple 15 are shown asjunction 20 and terminals 21 and 27. Junction 20 is maintained at thesame temperature (T₂) as the cold junctions 9 of thermopile 19. Thesecond junction, terminals 21 and 27, is at temperature T₀. The voltage(E_(c)) produced across terminals 21 and 27 as a result of these twothermocouple junctions can be represented by the relationship

    E.sub.c =α.sub.c (T.sub.2 -T.sub.0)                  (2)

where α_(c) is a Seebeck coefficient for the thermocouple materials andT₀ is the temperature of terminals 21 and 27, typically at ambienttemperature.

Since the thermopile 19 and the thermocouple 15 of FIG. 1B are connectedelectrically in series, the total output voltage E_(o) appearing acrossterminals 21, 27 of the circuit of FIG. 1 is the combination of thethermopile output voltage E_(p) and the thermocouple output voltageE_(c), Combining equation (2) and equation (6):

    E.sub.o =(1+c(T.sub.2 -T.sub.R))α.sub.pr 'e.sub.T ≐(T.sub.T.sup.4 -T.sub.2.sup.4)K+α.sub.c (T.sub.2 -T.sub.0)(8)

As seen by readout device 10, the final output voltage across terminals21 and 27 is the sum of the voltages produced by thermopile 19 andthermocouple 15 because they are connected in series. As shown in FIG.1B, the hot junction of thermocouple 15 is at the same temperature (T₂)as the cold junction temperature of the thermopile 19. As T₂ increases,T₁ -T₂ decreases and the thermopile output voltage Ep decreases. Butsince T₂ is also the hot junction temperature of thermocouple 15, theincrease in T₂ increases T₂ -T₀, and the thermocouple output voltageE_(c), also increases. Therefore, as long as the change in thermopileoutput voltage with temperature (dE_(p) /dT) is close to the change inthermocouple output voltage with temperature (dE_(c) /dT), thefluctuations in T₂ are negligible within a considerably wide range offluctuation. In other words, as long as

    |dE.sub.p /dT|≈|dE.sub.c /dT|

the decrease in E_(p) due to T₂ increasing is approximately equal to thecorresponding increase in E_(c), and the total output voltage E₀ =E_(c)+E_(p) is substantially unchanged. Proper selection of thermocouplematerial provides a value of α_(c) which makes the thermocouple voltageresponse which best compensates for changes in T₂.

The above analysis assumes that T₀ is kept constant, as is the case inthe preferred embodiment. However, T₀ may also be monitored and used tocompensate the output E₀ accordingly, if T₀ is allowed to fluctuate.Since T₀ is remote from the thermopile assembly, controlling and/ormonitoring T₀ is a simple task. In fact, conventional thermocoupleelectronics include a stable temperature reference for the cold junctionand the disclosed thermopile/thermocouple is compatible with suchelectronics.

According to the foregoing, the detector 12 provides an accuratetemperature indication of target temperature from sensed radiationwithout the use of complex electronics or cumbersome calculations. Thetemperature detector of the present invention is confined to thetemperature range of interest which allows an output signal to beprovided which is a substantially linear function of sensed radiation,thus satisfying the requirements of a standard output meter. Since thetemperature To can be kept remote from the thermopile sensor, it caneasily be held constant or measured separately. Since the thermalresponses of the thermopile and thermocouple are close to one another ina temperature range of interest, changes in thermopile cold junctiontemperature are compensated for by the thermocouple and no measurementof the cold junction temperature of the thermopile is necessary. Thisremoves the need for temperature sensors and/or heaters near thethermopile sensor.

The series connection of thermopile 19 and thermocouple 15 allows atarget temperature T_(T) to be measured relative to a referencetemperature T₀ which may be remote from the thermopile. The location ofterminals 21 and 27 may be selected to best suit the measuring purposes.The remote reference temperature removes the problems of isolating thecold junction reference temperature of a typical thermopile from ambienttemperature and the heat of the target. Very often, radiation sensorsmay be too close to or actually touch the target surface, and grossinaccuracies result due to fluctuation of the cold junction temperature.

Since the slope of the fourth power curve 14 of FIG. 2 changes quicklywith target temperature, it is important to select a temperature rangeof interest in which target temperatures are expected to fall. Thethermopile output is then calibrated with potentiometer 17 such that itsoutput is equal to a desired output voltage at a temperature in thecenter of the selected temperature range. This allows the thermopileoutput in the vicinity of the calibration temperature to appearapproximately linear to an output device responsive to a linear voltageoutput in that temperature range. For example, the 200° F. calibrationline 13 of FIG. 2 shows a linear function which intersects the curve 14at the 200° F. point. This intersection point represents the calibrationof the thermopile output for an expected target temperature of 200° F.

In a preferred embodiment of the invention, the 200° F. calibration isaccurate for target temperatures between about 175° F. and about 225° F.A second calibration, shown by the intersection of curve 14 with line 29of FIG. 2, is centered at 300° F. and is only accurate for targettemperatures between about 275° F. and about 325° F. Other calibrationsare also illustrated by the straight solid lines in FIG. 2.

Several examples of typical calibrations and resulting outputs of thepresent invention both with and without thermocouple 15 are shown in thefollowing tables. table represents a different application for whichresent invention might typically be used.

    ______________________________________    T.sub.T           T.sub.TM                   T.sub.2 K    T.sub.R                                     T.sub.o                                         α.sub.pr '                                               α.sub.c                                                   α.sub.cm    ______________________________________    TYMPANIC TEMPERATURE SENSOR    With Thermocouple    98.6   98.60   90      .881 90   70  30    30  30    98.6   98.58   60      .881 90   70  30    30  30    98.6   98.64   110     .881 90   70  30    30  30    Without Thermocouple    98.6   98.6    70      .86  70   70  30    0   30    98.6   108.62  60      .86  70   70  30    0   30    98.6   58.7    110     .86  70   70  30    0   30    INDUSTRIAL TEMPERATURE SENSOR    With Thermocouple    500    500     90      .245 90   70  30    30  23    500    509     40      .245 90   70  30    30  23    500    490     150     .245 90   70  30    30  23    Without Thermocouple    500    500     70      .335 70   70  30     0  30    500    546     90      .335 70   70  30     0  30    500    623     150     .335 70   70  30     0  30    HIGH TEMPERATURE APPLICATION    With Thermocouple    2000   2000    70      .169 70   70   1    30  4.9    2000   1979    40      .169 70   70   1    30  4.9    2000   2055    150     .169 70   70   1    30  4.9    Without Thermocouple    2000   2000    70      .169 70   70   1     0  4.9    2000   2163    40      .169 70   70   1     0  4.9    2000   1565    150     .169 70   70   1     0  4.9    ______________________________________

As demonstrated by the above tables, the proper selection of α_(pr) 'and the fine-tuning adjustment of K allows the displayed temperatureoutput T_(TM) of the detector to equal the actual target temperatureT_(T) for a given local temperature T₂. The meter used has a Seebeckcoefficient calibration of α_(cm). The stability of T_(TM) withfluctuations in T₂ is greatly enhanced by the use of a thermocouple.This improvement is demonstrated by the reduced fluctuation of T_(TM)with T₂ in the above tables when the thermocouple is used as compared towhen it is not used. Because the change in output signal of thethermopile with T₂ is inversely related to the change in the outputsignal of the thermocouple with T₂, the lower thermopile output signaldue to rising T₂ is compensated for by an increased thermocouple outputsignal.

Besides achieving the desired calibration of the thermopile output, itis also desirable to improve the linearity of the fourth powerrelationship within the chosen temperature range of interest. Asapparent from the curves of FIG. 2, the nonlinearity of the thermopilecurve 14 increases with increasing target temperature. In order toincrease the temperature range for which calibration is accurate,Applicant employs the following.

From the Stefan-Boltzmann relationship, the voltage output of thermopile19, which is linearly related to sensed radiation, is nonlinearlyrelated to target temperature. However, by filtering out shortwavelengths of thermal radiation from the target (below about 7microns), Applicant converts a region of the non-linear output from thethermopile 19 into one which is a better linear approximation. Thisresult is achieved by using the fact that long wavelength radiationmakes up a large percentage of the radiation emitted from the target atlower temperatures, but decreases relative to higher wavelengths astarget temperature is increased above about 200° F. By using a window 11with the circuit of FIG. 1B which is a material that filters out shortwavelength radiation, the percentage of the total radiation emittedwhich actually reaches the thermopile decreases as target temperatureincreases. This relationship is illustrated by the curve 16 shown inFIG. 3.

The curve 16 is a representation of the relative portion of the totalemitted radiation reaching the thermopile. As target temperatureincreases, the percentage of long wavelength radiation making up thetotal emitted target radiation decreases. Correspondingly, thepercentage of short wavelength radiation increases proportionally.However, the short wavelength radiation is blocked by the window filter11 and does not reach the thermopile. Therefore, the 4th power curve,which is a function of sensed radiation, is flattened out at highertarget temperatures to give the curve 65 shown in FIG. 2. The curve 65thus represents the product of curve 14 and curve 16. The fourth powercurve 14 is also shown in FIG. 3 so that the two curves 14, 16 can becompared. As shown by curve 65, the flattened fourth power thermopilecurve more closely approximates a linear function to increase theaccurate target temperature ranges.

To get the desired long wavelength pass (LWP) filtering of targetradiation reaching thermopile 19, window 11 is preferably formed of amaterial such as silicon which passes radiation of long wavelengths(about 7 microns to 20 microns) and filters out radiation of shortwavelengths (below about 7 microns). Other filter materials may also beused which have other wavelength cutoffs, and can otherwise change theshape of the fourth power curve.

The terminals 27 and 21 of thermocouple 15 are preferably adaptable toremovable connections to various readout devices. Thus, the voltageacross terminals 27, 21 is indicative of sensed radiation and may besupplied to various readout devices for displaying an indication ofsensed temperature. Different lead wires for thermocouple 15 may be usedfor different readout devices. For example, lead wire 25 may be iron andlead wire 23 may be constantan for a J type readout meter. In this case,the final output voltage of the detector is preferably 30 μV/°F. oftarget temperature above about 70° F. Or, in another example, wires 23,25 may be platinum and platinum RH, respectively, for readout meters oftypes R and S.

Common readout devices require input impedances higher than theimpedance of potentiometer 17. If, however, input impedance to thereadout device is of the same order as the potentiometer 17 impedance,then the range of target temperatures for which detector 12 is accuratemay be further increased. Also, for detector configurations that areaccurate below about 200° F., the detector is more stable if smallinternal resistance-capacitances and relatively large externalresistance-capacitances are employed. Because the potentiometer 17,filter window 11 and receiving readout devices 10 are adjustable, thedetector 12 is a versatile tool for detecting surface temperature orheat loss in various applications.

A different embodiment of the present invention is shown in FIG. 4. Inthe configuration of FIG. 4, each of two thermopiles 101, 103 is wiredin series with its own thermocouple 102, 104. Therefore, in essence, twoversions of the detector of FIG. 1B are provided but are wireddifferentially at their remote reference temperature outputs T_(0A) andT_(0B). In FIG. 4, the top sensor is referred to as sensor A and thebottom sensor is referred to as sensor B. Using the calculationsdiscussed with relation to FIGS. 1A and 1B, the output voltage V_(A)across terminals 106, 107 approximately equals T_(1A) -T_(0A), and istherefore indicative of the temperature difference between the hotjunction of thermopile assembly 101 and reference temperature T_(0A).Similarly, the output voltage V_(B) across output terminals 108, 109 ofsensor B approximately equals T_(1B) -T_(0B), and is indicative of thetemperature difference between the hot junction of thermopile 103 andreference temperature T_(0B). The thermopiles 101 and 103 as well as thethermocouples 102, 104 are well matched such that the outputcharacteristics of sensor A and sensor B are as close as possible.

Since T_(0A) and T_(0B) may be remote from their respective thermopilesensors, they are easily held at the same reference temperature suchthat T_(0A) =T_(0B) =T₀. This is easily accomplished by locating thereference temperatures T_(0A) and T_(0B) in close proximity to oneanother or by thermally connecting them. Both sensor A and sensor B aretherefore referenced to the same remote reference temperature T₀. Thus,their outputs may be compared in a differential manner. As shown in FIG.4, terminal 106 of sensor A is wired to terminal 108 of sensor B toprovide a common electrical reference. Terminal 107 of sensor A andterminal 109 of sensor B are then used as a differential output, andwill be equal to V_(A) -V_(B).

Since

    V.sub.A ≈α.sub.A (T.sub.1A -T.sub.0)

and

    V.sub.B ≈α.sub.B (T.sub.1B -T.sub.0)

    V.sub.D =V.sub.A 31 V.sub.B ≈α.sub.A T.sub.1A -α.sub.A T.sub.0 -α.sub.B T.sub.1B +α.sub.B T.sub.0

But since the two sensor circuits A and B are very closely matched,α_(A) ≈α_(B) ≈α, and

    V.sub.D ≈α T.sub.1A -T.sub.0 -T.sub.1B +T.sub.O !=α T.sub.1A -T.sub.1B !

Therefore, V_(D) is an accurate representation of the difference intemperature between the target of sensor A and the target of sensor B.

Because the arrangement of FIG. 4 allows each sensor to be referenced toremote temperature T₀ separately, it is not necessary that the coldjunctions of thermopile 101 and 103 be thermally connected or proximateto one another. This provides a distinct advantage in allowing thedifferential sensor 100 to measure a difference in temperature betweentargets which are quite a distance from each other. The location,position and orientation of the sensors relative to one another does notaffect the sensing as long as they have a common remote referencetemperature T₀. No comparison of thermopile cold junction temperaturesis necessary, and much of the complicated heating and measuringcircuitry of past differential thermopile sensors may be omitted. Itshould be recognized that the remote reference temperature may belocated anywhere including at one of the local reference temperaturesT₂, but is generally most conveniently located at remote electronicsremoved from the sensed environment.

One notable use of the sensor design shown in FIG. 1B is as a tympanictemperature monitor to sense the temperature of a person's tympanicmembrane, which is close in temperature to body core temperature. Oftenit is necessary or desirable to monitor a person's body temperature,such as during a surgical operation. Since no complex heaters orelectronics are necessary in the region of the thermopile sensor, thepackage in which it is housed is very small and simple. The design ofFIG. 1B thus allows a sensor to be inserted directly into a subject'sear canal with only lead wires leading out to an output device. Such adesign would also be applicable to an ambulatory tympanic temperaturemonitor as well.

In the aforementioned embodiments, the thermopile and the thermocoupletogether form a detector suitable for applications involving an expectedmean target temperature and a known thermopile cold junction temperaturerange. However, since the linear thermal response of the thermocouple isemployed to compensate for the non-linear thermal response of thethermopile, the thermopile cold junction temperature range must be knownand relatively narrow.

Referring to FIG. 5A, another embodiment of the present inventioncomprises a radiation detector having a temperature dependent variableresistor 216 coupled to the thermopile 19 and providing a variableresistance that combines with the thermopile output voltage to produce alinearized thermopile output voltage. As such, the thermopile output,linearized by the thermistor, combines the linear thermocouple outputfrom thermocouple 231 to provide an detector output Eo that is stableover a broad range of thermopile cold junction temperatures.

The radiation detector of FIG. 5A has a thermopile 19 which sensesradiation emitted from a target entering through a window 211 andprovides an output signal indicative of the target temperature. As inprevious embodiments, a thermocouple 231 is connected electrically inseries with the thermopile 19 and thermally coupled thereto. Since thethermocouple output changes linearly with changes in the thermopile coldjunction temperature while the thermopile output signal changesnon-linearly with said changes in the temperature of the thermopile coldjunction, the thermocouple 231 provides temperature compensation for asomewhat narrow range of thermopile cold junction temperatures.

In accordance with another aspect of the present invention, a negativetemperature coefficient (NTC) thermistor 216 is electrically connectedin series with the thermopile 19 and thermally coupled to the coldjunction 9 of the thermopile, providing a variable resistance thatcombines with the thermopile output voltage to produce a linearizedoutput voltage with cold junction temperature. The linearized thermopileoutput voltage combines with the thermocouple output voltage to producea stable total output voltage over a broad range of thermopile coldjunction temperatures. Note that a positive temperature coefficient(PTC) thermistor 316 may be connected in parallel with the thermopile,as shown in FIG. 5B, to achieve the same result.

Recall from equation (8) that the thermopile output voltage (E_(p))changes nonlinearly with changes in T_(T) and T₂ due to the presence ofthe fourth power term (T_(T) ⁴ -T₂ ⁴). This fourth power term isdemonstrated by the curve 234 of FIG. 6A which shows how the thermopileoutput voltage increases with increasing target temperature for a fixedthermopile cold junction temperature. The fourth power term is furtherdemonstrated by the curve 236 of FIG. 6B which shows how the thermopileoutput voltage decreases with increasing thermopile cold junctiontemperature for a fixed target temperature. Also shown in FIG. 6B iscurve 246 which demonstrates how the thermocouple output voltage (E_(c))increases linearly with increasing thermopile cold junction (i.e.,thermocouple hot junction) temperature.

Because of the inverse relationships of the thermopile and thermocoupleoutputs with change in cold junction temperature, the combined outputscan be made generally independent of cold junction temperature. To thatend the thermopile output 236 is linearized to approximate a linearcurve 238. The curve 238 combines with the thermocouple curve 246 toprovide an output 248 which is generally constant with changes in coldjunction temperature. To provide the linearized curve 238 over the coldjunction temperature range of interest T_(N) to T_(X), the thermopilecurve 236 should be multiplied by a curve 249. As illustrated in FIG. 6Cwhere the Y axis is expanded, that curve 249 is approximated by a curve240 in a temperature range of interest T_(N) to T_(X). The curve 240 isthe result of a thermistor circuit as described below.

Referring back to FIG. 5A, the NTC thermistor 216 is connected to a pairof resistors R1 (at 222) and R2 (at 224) which combine with thethermistor to provide a variable resistance R(T₂) having a responsecurve 240 shown in FIG. 6C. The curve 240 has a maximum resistance and aminimum resistance. Since the thermistor 216 and resistors 222, 224 areelectrically connected in series with the thermopile 19 and thermallycoupled to the thermopile cold junction 9, the resistance R(T₂) combineswith the thermopile output voltage (E_(p)) to produce an output voltage(E_(p1)) that changes approximately linearly with thermopile coldjunction temperature.

Referring back to FIG. 5A, a thermocouple 231 is connected in serieswith the thermopile 19 and the thermistor 216 in a manner such that afinal output voltage (E_(o)), indicative of sensed radiation of atarget, is provided across the ends of the lead wires 231 and 233 of thethermocouple 231. These ends are connected to respective terminals 241and 242 of readout device 10 which uses E_(o) to provide an indicationof target temperature. For a given target temperature, the thermocouple231 provides compensation to the linearized thermopile output signal forchanges in thermopile cold junction temperature T₂ such that the finaloutput voltage (E_(o)) remains constant. Rather than forcing T₂ to equala particular temperature, or measuring T₂ to use in calculating thefinal output voltage, the thermocouple 231 automatically compensates forchanges in the linearized output (E_(p1)) with fluctuations in T₂.

Thermocouple 231 of FIG. 5A is made of wire 232 and wire 233. Wire 232as shown by the broken line is of a different metal than wire 233.Effectively, the two junctions of thermocouple 231 are shown asterminals 241 and 242 and terminals 243 and 244. The first junction, atterminals 243 and 244, is at the thermopile cold junction temperature.The second junction, at terminals 241 and 242, is at temperature T₀.Recall from equation (2) that the voltage (E_(c)) produced acrossterminals 241 and 242 as a result of these two thermocouple junctionscan be represented by the relationship

    E.sub.c =α.sub.c (T.sub.2 -T.sub.0)                  (9)

where α_(c) is a Seebeck coefficient for the thermocouple materials andT₀ is the temperature of terminals 241 and 242, typically at ambienttemperature.

Since the thermopile 19, the thermistor 216 and the thermocouple 131 ofFIG. 5A are connected electrically in series, the total output voltageE_(o) appearing across terminals 241 and 242 is the combination of thelinearized thermopile output voltage E_(p1) and the thermocouple outputvoltage E_(c),

    E.sub.o =E.sub.p1 +E.sub.c                                 (10)

Further, since the thermocouple hot junction is at the same temperatureas the thermopile cold junction, the thermocouple output voltageresponse is inversely related to the linearized thermopile outputvoltage response for changes in T₂. Thus, referring to FIG. 6B, thethermocouple voltage response 246 is inversely related to the linearizedthermopile output voltage response 238 for changes in the thermopilecold junction temperature range. As T₂ increases, the thermopile outputvoltage E_(p) decreases. But since T₂ is also the hot junctiontemperature of thermocouple 231, the increase in T₂ causes thethermocouple output voltage E_(c) to increase such that fluctuations inT₂ have a negligible effect on the total output signal 248. Thus, aslong as the decrease in E_(p1) due to T₂ increasing is equal to thecorresponding increase in E_(c), the total output voltage response 248(E_(o) =E_(c) +E_(p)) is substantially unchanged.

The above analysis assumes that T₀ is kept constant, as is the case inthe preferred embodiment. However, T₀ may also be monitored and used tocompensate the output E_(o) accordingly, if T₀ is allowed to fluctuate.Since T₀ is remote from the thermopile assembly, controlling and/ormonitoring T_(O) is a simple task. In fact, conventional thermocoupleelectronics include a stable temperature reference for the cold junctionand the present invention is compatible with such electronics. By usingthe thermocouple in series with the thermopile, the remote controlledthermocouple reference T₀ becomes the thermopile reference temperature.As such, there is no need to monitor the internal cold junctiontemperature of the thermopile.

Referring once again to FIG. 5A and FIG. 6B, a potentiometer may beemployed to fine-tune the linearized thermopile output response 238 tointersect a thermocouple output response 246 at a desired targettemperature to produce a stable total output E_(o) over a broadthermopile cold junction temperature range. More specifically, apotentiometer 218 scales the linearized thermopile output E_(p1),allowing the detector to be fine-tuned to an expected targettemperature. This ensures that the total output signal E_(o) is asaccurate as possible within a range around the target temperature.

Having the manually adjustable potentiometer 182 also allows a user ofthe detector to correct for any variations of the detector due tomanufacturing tolerances or other influences. For example, thermopileshave parameters, such as characteristic impedance R_(s), that varysignificantly from device to device. The potentiometer may be adjustedto compensate for these variations such that a number of detectorsprovide the same total output response for a given range of thermopilecold junction temperatures. The presence of potentiometer scaling factorK in equation (8) shows that the linearized output voltage E_(p1) can becontrolled by controlling the setting of the potentiometer.

Referring to FIG. 5A, both the thermopile and the output device 10 arehigh impedance devices, so the radiation detector acts as an antennareceiving stray high frequency noise which distorts the measurement bythe output device 10. Accordingly, the radiation detector also comprisesa capacitor 228 connected in parallel to its output leads to attenuatehigh frequency noise, especially noise at and above 60 Hz. Preferably,the capacitor has a value of 1-5 μf. At low frequencies, the capacitor228 has no effect on the output impedance of the radiation detector asit resembles an open circuit. However, at high frequencies, thecapacitor approaches a short circuit, such that radiation detectoroutput impedance becomes low, thereby attenuating high frequency noiseat the output device 10.

Referring to FIG. 7, in accordance with another aspect of the presentinvention, the thermocouple 231 may comprise a non-intersecting pair ofleads formed of different materials and coupled to a thermopile circuitsuch that the thermopile circuit actually serves as the hot junction ofthe thermocouple. As explained previously, the thermocouple 231 isformed of two wires 232 and 233 formed of dissimilar metals. Since thethermocouple is electrically connected in series with the thermopile,both thermocouple wires are electrically connected at differentlocations to the thermopile circuit. More specifically, the firstthermocouple lead 232 is electrically connected at junction 244 to oneof the thermopile leads which is connected to a thermopile circuit andtherefore held at the cold junction temperature of the thermopile.Although the second thermocouple lead 233 is not electrically connecteddirectly to a thermopile lead, it is electrically connected at junction243 to the thermopile circuit. Further, the second thermocouple lead ismounted in close proximity to the thermopile and thermally coupled tothe cold junction of the thermopile with epoxy 239. With boththermocouple leads held at the same temperature, the temperature of thethermopile cold junction, the leads do not have to intersect to providea thermocouple hot junction.

Referring to FIG. 8, yet another embodiment of the present inventioncomprises a radiation detector having a thermopile 19 and a thermistor216 and providing a linearized thermopile output voltage over thermopilecold junction temperature. This embodiment does not include athermocouple. Since the linearized thermopile output is a linearfunction with changes in the thermopile cold junction temperature, theoutput detected by a meter 10 only requires a linear compensation forcold junction temperature. In prior embodiments, the thermocouple is thelinear compensation element. However, more conventional thermopilesystems sense the cold junction temperature, as with a temperaturedetecting thermistor, and then provide compensation to the signaldetected by the meter. A linear compensation is generally easier tomake.

FIG. 9 illustrates yet another embodiment of the radiation detectorcircuit. In this circuit, the thermopile 19 is shown as its electricalequivalents of a voltage source V_(TP) and in internal resistanceR_(TP). A user adjustable potentiometer 300 is coupled directly acrossthe thermopile 19 to allow for calibration of the thermopile output. Asbefore, a temperature dependent resistor 302, in cooperation withparallel resistor 304 and series resistor 306, make the output takenacross resistor 306 generally independent of cold junction temperature.With the potentiometer 300 coupled directly across the thermopile ratherthan at the output leads, the output impedance of the circuit can beminimized while also minimizing the sizes of the resistors and thus ofpower losses in the circuit.

From equation 3, it can be seen that the output voltage of thethermopile is directly dependent on the emissivity of the targetsurface. Accordingly, very low emissivity materials such as shiny metalpresent particular difficulties in using radiation detectors.Reflections from the environment should be minimized as by using aconical cup about the sensor aperture as suggested in U.S. Pat. No.4,636,091. Because of the low signal received from the low emissivitytarget, a relatively large thermopile having a large Seebeck coefficientis required to provide a suitable output. A further difficulty ispresented by the fact that the emissivity is itself a function oftemperature, and changes in emissivity with temperature becomesignificant with the lower levels of detected emissions. Thus, itbecomes even more important that the thermopile include a calibrator toenable the output signal to be adjusted to provide a desired outputresponse at a desired target temperature from a target surface of aparticular emissivity.

The effect of emissivity changing as a function of temperature can bereduced by using a filter 11 which limits the sensed radiation toshorter wavelengths. At less than 5 microns, the change in thermopileoutput with temperature is significantly greater than the change inthermopile output with emissivity. Unfortunately, just as the selectionof long wavelengths serves to flatten the detector response, theselection of shorter wavelengths increases the nonlinearity of thedetector response. Accordingly, it becomes even more critical that thedevice be user calibrated to a particular target temperature, since theadded nonlinearity significantly reduces the range of accuratemeasurement.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A method of non-contact temperature detectioncomprising:viewing radiation from a target surface with a thermopile;scaling the output of the thermopile to approximate output of athermocouple in contact with a surface at the temperature of the targetsurface; and with a thermocouple meter, providing a temperatureindication from the scaled thermopile output.
 2. A method as claimed inclaim 1 wherein the output of the thermopile is scaled by a resistancein parallel with the thermopile.
 3. A method as claimed in claim 2wherein the resistance is a user variable resistance.
 4. A temperaturedetector comprising:a thermopile adapted to receive radiation from atarget surface; and a passive scaling circuit which scales the output ofthe thermopile to approximate output of a thermocouple in contact with asurface at the temperature of the target surface.
 5. A temperaturedetector as claimed in claim 4 wherein the scaling circuit comprises aresistance coupled parallel to the thermopile.
 6. A temperature detectoras claimed in claim 5 wherein the variable resistance is a user variableresistance.